Understanding SF6 Gas Pressure and Phase Changes in Industrial Cylinders

[Ian_Brooks]

I'm trying to get my head around a concept.

We use Industrial Gas cylinders filled with SF6 gas - the gas cylinder is 70-80% liquid and the rest is in a gaseous state.

If this is kept at a pressure of approx 40Bar and is passed through a regulator that allows the gas to be filled into a containment unit at 1bar -

How does the liquid SF6 become a gas when kept at the same temperature? Obviously the pressure inside the cylinder will decrease as the gas is released.

I realize the liquid freezes if the pressure drop inside the cylinder is too fast.

The pressure curve is as follows.
http://img80.imageshack.us/img80/4661/sf6pressuretempliq.jpg

Datasheet available here:
http://www.solvaychemicals.com/docroot/fluor/static_files/attachments/sf6_e.pdf

 

[russ_watters]

40 bar is off the top of the graph...at room temperature the gas/liquid mixture must be at about 22 bar. And as you release some, the pressure drops and you go from the border of the liquid/gas region into the gas region, so some liquid boils until you get back to equilibrium.

 

[Ian_Brooks]

Thanks,

Inside the cylinders we keep them at a higher pressure when compared to that graph

 

[Darmog]

The extra gas is at same temperature as liquid but has gained energy from its suroundings. The liquid will have cooled very slightly but this is compensated by gaining some heat (energy) from the cylinder case and then from the environment. (energy is conserved)
The latent heat factor may be such that temporary cooling of liquid not measurable in the practical situation.
Darmog

 

[russ_watters]

Thanks,

Inside the cylinders we keep them at a higher pressure when compared to that graph

So then you keep it as a compressed liquid, not a mixture?